bug 5301: improve escaping of XML characters in bib output

bug 5301: improve escaping of XML characters in bib output

<, >, ', or " in an item call number will no longer make
the bib displays break when using XSLT mode.

Added a new routine to C4::Koha, xml_escape(), to implement
converting &, <, >, ', and " to their corresponding
entities.

Patch loosely based on work done by Daniel Latrémolière <daniel.latremoliere@bulac.fr>

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>